The long-term objective of this work is to understand the molecular basis by which cells are polarized. Polarity is a fundamental property of cells that is required for proper development as well as adult physiology. For example, during development cell fate determinants are polarized in dividing cells as a mechanism for generating cell type diversity and the loss of cell polarity is a hallmark of many disease states, including cancer. For spatially and temporally precise establishment of cell polarity to occur, cellular signals must be interpreted and ultimately coupled to the segregation of the relevant cellular components. In diverse cell types, polarity is controlled by the evolutionarily conserved Par complex consisting of Bazooka (Baz; aka Par-3), Par-6, and atypical Protein Kinase C (aPKC). It has recently become clear that aPKC activity is the primary output of the Par complex. In this grant we seek to answer three fundamental aspects of aPKC-how it is targeted to specific cellular sites, how its activity is regulated, and how phosphorylation of substrates regulate their localization. Understanding the molecular events that lead to the coupled recruitment and activation of the Par complex will yield new insight into cell polarity.